The purpose of this study is to evaluate the prophylactic use of oral hypoglycemic drugs in subjects with asymptomatic diabetes in preventing or postponing the progression of diabetes to its overt symptomatic stage and its vascular and neurologic complications. The availability of a large and stable population cared for by the Kaiser Health Plan in Oakland and San Francisco provides a unique opportunity to study this problem. A long term double blind study of 350 subjects, employing tolbutamide, phenformin and a placebo has been in progress since 1964. Glucose tolerance tests, including measurements of plasma immuno-reactive insulin and free fatty acids, pulse wave velocity, motor and sensory nerve conduction velocity, and a broad battery of medical tests of the Kaiser-Permanente Automated Multiphasic Screening Program, are being done serially to evaluate the response of the research subjects. The mortality experience by cause and the occurrence of non fatal cardiovascular events are being carefully reviewed to determine whether differences are occurring between the treatment groups. The control or placebo treated groups will permit a better understanding about the natural history of this stage of diabetes. Data regarding glucose tolerance and plasma levels of insulin, free fatty acids and vascular and neuropathic findings in a group of diabetic patients will provide information about characteristics of the diabetic state, changes in these characteristics with time and the correlation of these parameters.